Process for preparing 1,1,1-trifluoro-2-chloroethane and/or 1,1,1,2-tetrafluoroethane

ABSTRACT

A process for preparing 1,1,1-trifluoro-2-chloroethane and/or 1,1,1,2-tetrafluoroethane, comprising the steps of reacting trichloroethylene and/or 1,1,1-trifluoro-2-chloroethane with hydrogen fluoride in the presence of a fluorinating catalyst in a gas pase, separating 1,1,1-trifluoro-2-chloroethane, hydrogen fluoride, and optionally trichloroethylene, from the reaction mixture obtained by the above reaction which contains 1,1,1-trifluoro-2-chloroethane, 1,1,1,2-tetrafluoroethane, hydrogen fluoride, and optionally trichloroethylene, and recycling them to the reaction step above, characterized in that at least a part of the reaction mixture is distilled in a distillation tower, 1,1,1,2-tetrafluoroethane-rich components are discharged from the top of the tower, and a mixture of 1,1,1-trifluoro-2-chloroethane and hydrogen fluoride, and optionally trichloroethylene, is discharged in the form of gas from the middle part of the tower at a temperature which is above the boiling point of the azeotropic mixture of 1,1,1-trifluoro-2-choroethane and hydrogen fluoride and at least 5° C. below the boiling point of anhydrous hydrogen fluoride under the internal pressure in the distillation tower and returned to the reaction step. 
     The discharging at the middle part of the tower provides a recycling gas in high purity, while leaving higher-boiling substances such as water, tar, etc. on the bottom of the tower. The gas can be used for the reaction system as a recycling gas, which can eliminate a process of vaporizing high-boiling mixture which is conventionally discharged at the bottom of the tower by means of a vaporizer and the like and reduce the equipment cost significantly.

FIELD OF THE INVENTION

The present invention relates to a process for preparing1,1,1-trifluoro-2-chloroethane (referred to as R-133a hereinafter)and/or 1,1,1,2-tetrafluoroethane (referred to as R-134a hereinafter),which comprises reacting trichloroethylene and/or R-133a with hydrogenfluoride (referred to as HF hereinafter) in the presence of afluorinating catalyst, and separating R-133a, HF, and optionallytrichloroethylene, from the reaction mixture, and recycling them to thereaction process.

R-133a is useful as an intermediate of fluorinated organic compounds.R-134a is used as a coolant and a blowing agent as a substitute fordichlorodifluoroethane which is one of the recently regulatedhalocarbons.

DESCRIPTION OF THE PRIOR ART

R-133a and R-134a are usually produced by fluorinating trichloroethylenewith HF in a gas phase in the presence of a catalyst which compriseschromium. R-134a is also produced from R-133a and HF by a similar way tothe above method. These reactions proceed stepwise and are shown by thefollowing scheme: ##STR1##

There is a reverse reaction of from the right side to the left side inthe reaction (2) which provides R-134a. Moreover, it proceeds about 100times as fast as the reaction of from the left to the right. Therefore,HF must be used in large excess even in a relatively efficient gas phasereaction. However, it is difficult to obtain R-133a in a conversion ofmore than 30%. Accordingly, it is necessary to separate unreacted R-133aand HF from the reaction mixture and to recycle them to the reactionprocess. However, the prior arts only describe a method in which thewhole reaction mixture is washed with water. There has been known nomethod in which the unreacted R-133a and HF can be efficiently separatedand recycled.

The reaction mixture obtained in the process comprising the gas phasereaction contains R-133a, R-134a, hydrogen chloride, HF, and optionallytrichloroethylene. When trichlroethylene is completely fluorinated inthe reaction process, or when R-133a is fluorinated, substantially notrichloroethlene is contained in the reaction mixture. However, even inthose cases, the reaction mixture is seldom completely free fromtrichloroethylene and usually contains as much as 0.1 mole %trichloroethylene. In addition, the mixture inevitably contains a smallamount of water which is present in the starting materials. Water, evenin a small amount, lowers the activity of the catalyst used and preventsthe reaction.

In a usual way, the reaction mixture which has been cooled is charged toa distillation tower. The low-boiling mixture consisting of hydrogenchloride which shifts the equlibrium of the reaction (2) to the leftside and an azeotropic mixture of R-134a and HF is discharged from thetop of the distillation tower, and the high-boiling mixture, consistingpredominantly of R-133a and HF, and optionally trichloroethylene, isdischarged from the bottom of the tower. Thereafter, the high-boilingmixture is vaporized with a vaporizer and recovered as a recycling gasin the reaction system. In this method, a vaporiser is required inaddition to the distillation tower, which makes equipment verycomplicated. Moreover, in this method, the small amount of water whichis contained in the reaction mixture is also recyled to reaction stepand accumulated in the reaction system. Thus the method in which R-133aand HF are separated from the reaction mixture and recycled to thereaction step has the problems that impurities are incorporated intoreaction system, and that a great deal of equipment to provide heat isrequired.

SUMMARY OF THE INVENTION

The inventors have made intense studies to solve the above problems andfound the following facts: R-133a and HF, and trichloroethylene and HFform azeotropic compositions respectively, which, moreover, are minimumboiling mixtures, different from the azeotropic composition of water andHF; under a certain pressure, a mixture of any composition consisting ofHF, R-133a and trichloroethylene has an equilibrium temperature betweenthe boiling point of the azeotropic composition of R-133a and HF and theboiling point of HF; since the temperature is remote from the azeotropicpoint of water and HF, a mixture of the above three compounds can beeasily obtained in a high purity. The inventors have further found that,when the reaction mixture of the fluorination is distilled to separatethe components, a gaseous mixture which has been separated anddischarged from the middle part of the distillation tower can berecycled to the reaction step, which enables the omission of vaporizingfacilities usually required.

Accordingly, the present invention provides a process for preparingR-133a and/or R-134a, comprising the steps of reacting trichloroethyleneand/or R-133a with hydrogen fluoride in the presence of a fluorinatingcatalyst in a gas phase, separating R-133a, hydrogen fluoride, andoptionally trichloroethylene, from the reaction mixture containingR-133a, R-134a, hydrogen fluoride, and optionally trichloroethylene,which are obtained by the above reaction, and recycling R-133a, hydrogenfluoride, and optionally trichloroethylene to the reaction step,characterized in that at least a part of the reaction mixture isdistilled in a distillation tower, R-134a-rich components are dischargedfrom the top of the tower, and that a mixture of R-133a, hydrogenfluoride, and optionally trichloroethylene, is discharged in the form ofgas from the middle part of the tower at a temperature which is abovethe boiling point of the azeotropic mixture of R-133a and hydrogenfluoride and at least 5° C. below the boiling point of anhydroushydrogen fluoride under the internal pressure in the distillation towerand returned to the reaction step.

DETAILED DESCRIPTION OF THE INVENTION

The process according to the present invention can be applied to anycomposition used in the reaction of trichloroethylene, R-133a or both ofthem with HF in a gas phase.

The reaction mixture obtained by the reaction process is a gascontaining mainly R-133a, R-134a, hydrogen chloride and HF, andoccasionally trichloroethylene. They are cooled usually to a temperatureof from -50° C. to 50° C. and most of them are liquefied and charged toa distillation tower. However, a method for charging the reactionmixture to the distillation tower is not limited to the one mentionedabove. In addition to liquid charging, the gas which can not beliquefied may be charged to the distillation tower under, an elevatedpressure, if necessary, or the whole mixture may be charged theretowithout liquefication under an elevated pressure. In addition, thereaction mixture may be charged thereto in the form of a liquid, a gasor a mixed phase under the inherent pressure of the mixture.

The pressure in the distillation tower is not particularly limited, butis usually from atomospheric pressure to 20 atm. If the pressure islower than atomspheric pressure, a considerable amount of energy isrequired for cooling at the top of the tower. When the pressure exceeds20 atm., the expense for a pressure-proof distillation tower is needed.

The reaction mixture introduced into the distillation tower is separatedinto a low-boiling mixture containing mainly hydrogen chloride, R-134aand HF, and a high-boiling mixture containing mainly R-133a,trichloroethylene and HF. The low-boiling mixture is discharged from thetop of the tower and the high-boiling mixture is discharged from amiddle part of the tower which is positioned below the charging part ofthe reaction mixture and above the bottom of the tower. The dischargingtemperature should be above the azeotropic point of R-133a and HF, andat least 5° C. below the boiling point of HF. Otherwise, R-134a, water,etc. are circulated in the reaction process. The azeotropic points ofR-133a and HF are 20° C. and 66° C. under pressures of 5 kg/cm² G and 9kg/cm² G, respectively. The boiling points of HF are 47° C. and 102° C.under pressures of 5 kg/cm² G and 9 kg/cm² G, respectively.Trichloroethylene has a boiling point of 120° C. under a pressure of 1.5kg/cm² G and therefore, in usual cases, remains on the bottom of thetower as the high boiling substance. However, since the azeotropic pointof trichioroethylene and HF is 41° C. under a presure of 1.5 kg/cm² G,it can be discharged in the above temperature range.

Discharge of the high-boiling mixture from the middle part of thedistillation tower provides a recycling gas of high purity, withremaining higher-boiling substances such as water, tar, etc. on thebottom of the distillation tower. The recycling gas as such can be usedfor the reaction system. Thus it is possible to eliminate theconvetional procedure of vaporizing the high-boiling mixture which hasbeen discharged from the bottom of the distillation tower with avaporizer. The simplification of facilities reduces the cost offacilities.

Water, tar, etc. which are dischgarged from the bottom of the towerusually contain HF. They may be distilled in a separate distillationtower in order to recover HF which is a useful substance.

The ratio of R-133a and HF, which are discharged from the middle part ofthe tower, has a great influence on the fluorinating reaction.Therefore, it can be suitably adjusted by adding HF to the distillationtower.

When R-133a is intended to be obtained alone, it can be discharged as aconstant boiling mixture of R-133a and HF at an upper part of the towerabove the part for discharging the recycling gas.

EXAMPLES Example 1

In a distillation column (the number of thoretical plates: 35, a columndiameter: 50 mm) having a 5 l vessel below it and a condenser above it,there were charged 2 moles of R-134a, 25 moles of R-133a and 70 moles ofHF, and a refluxed condition was made in such a manner that a pressureof 9 kg/cm² G was kept without discharging any component. At this time,the temperature at the top of the column was 39° C. The refluxing liquidhad a composition of 95 mole % of R-134a and 5 mole % of HF, which wasan azeotropic composition of R-134a and HF. A gaseous reaction mixtureof the reaction of R-133a with HF and/or trichloroethylene with HF wascooled to 0° C. Only the resulting liquid phase was introduced in thedistillation column with a pump. The flow rates are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        Gaseous Reaction Mixture                                                                          Liquid charged to Column                                  Com-   Flow rate Composition                                                                              Flow rate                                                                             Composition                               ponent (mol/hr)  (mole %)   (mol/hr)                                                                              (mole %)                                  ______________________________________                                        R-133a 87        19.9       84      22.0                                      R-134a 14        3.2        8       2.1                                       HF     294       67.3       288     75.4                                      HCl    42        9.6        2       0.5                                       Tri-   0.1       (0.02)     0.1     (0.03)                                    chloro-                                                                       ethylene                                                                      Water  0.01      20 ppm     0.01    30 ppm                                    Others 0.1       (0.02)     0.1     (0.03)                                    ______________________________________                                    

Since the pressure increased as the liquid was introduced, it was keptat 9 kg/cm² G by discharging a gas thorough the outlet of the condenser.An uncondesed gas was washed with water, dried, and liquified under anelevated pressure to store it in a container. After about 5 minutes,since the gas temperature in the neighborhood of the discharging outletat the third plate from the bottom of the column became 90° C.,discharging was started with keeping the volume of the liquid in thevessel constant. The discharged gas as such was returned to the reactoronly with the addition of HF and trichloroethylene consumed. Thisprocedure was continued. After 2 hours, an analysis was made for thecompositions of the recycling gas, the discharged gas at the top of thecolomn and the liquid in the vessel. At this time, the dischargingtemperature was 88° C.

The content of acidic components, i.e. HF and hydrogen chloride, wasdetermined as follows: At first, a sample was absorbed into water tomake an aqueous solution. The total content of acidic components wasdetermined by titrating the aqueous solution. On the other hand, theratio of chloride ion to fluoride ion was determined by ionchromatography. The individual contents of the acidic components werecalculated from the values determined above. Organic substances wereanalyzed by gas chromatography after washing with water and drying. Thewater content was determined by the Karl-Fisher method. The compositionsare given in Table 2. Table 2 shows that water can be removedefficiently, and the useful unreacted substances which had beenintroduced into the distillation tower, i,e. HF, R-133a andtrichloroethylene, can be recovered without any loss, and that they areof high purity.

Comparative Example 1

Example 1 was repeated except that the discharging was started when thetemperature of the discharging outlet became 65° C. The composition ofthe discharged gas are given in Table 2, which shows that the gas iscontaminated with R-134a.

Example 2 and Comparative Example 2

Example 1 was further continued. After 12 hours, the temperature of thedischaging outlet began to rise gradually. The compositions of thedischarged gases obtained when the temperatures of the dischargingoutlet were 95° C. and 100° C., were shown at the columns of Example 2(95° C.) and Comparative Example 2 (100° C.) in Table 2, respectively.

                  TABLE 2                                                         ______________________________________                                               Composition (mole %)                                                                      Com.              Com.                                     Component                                                                              Example 1 Exam. 1   Example 2                                                                             Exam. 2                                  ______________________________________                                        R-133a   20        30        10       5                                       R-134a   trace      1        trace   trace                                    HF       80        69        90      95                                       HCI      N/D       N/D       N/D     N/D                                      Trichloro-                                                                             (0.03)    trace     (0.03)  (0.03)                                   ethylene                                                                      Water    trace     trace     trace   25 ppm                                   Others   trace     trace     trace   trace                                    ______________________________________                                    

Example 3

Example 1 was repeated except that the discharging was effected at thevessel below the bottom of the tower in such a manner that a liquidvolume in it was kept constant, instead of discharging at the thirdplate from the botom of the tower. Water was contained in an amount of30 p.p.m. in the discharged liquid.

Effect of the Invention

As can be understood from the description above, according to thepresent invention, when the reaction mixture of the fluorinatingreaction is cooled and charged in a distillation tower, a mixture ofR-133a, trichloroethylene and HF containing no other impurity can beobtained at the middle of the lower part in the tower. In addition, thedischarged mixture can be recycled directly to the reaction process byonly heating it without a vaporizer, which enables the omission of agroup of vaporizing facilities and the simplification of equipment.Moreover, it is possible to concentrate water and tar on the bottom ofthe distillation tower and easily remove them from the reaction-recyclesystem.

What is claimed is:
 1. A process for preparing1,1,1-trifluoro-2-chloroethane and/or 1,1,1,2-tetrafluoroethane,comprising the steps of:(a) reacting trichloroethylene and/or1,1,1-trifluoro-2-chloroethane with hydrogen fluoride in the presence ofa fluorinating catalyst in a gas phase; (b) separating a recyclingmixture of 1,1,1-trifluoro-2-chloroethane, hydrogen fluoride, andoptionally trichloroethylene, from the reaction mixture, which contains1,1,1-trifluoro-2-chloroethane, 1,1,1,2-tetrafluoroethane, hydrogenfluoride, and optionally trichloroethylene; and (c) recycling themixture separated in step (b) to the reaction process wherein at least apart of the reaction mixture is distilled in a distillation tower,1,1,1,2-tetrafluoroethane-rich components are discharged from the top ofthe tower, and that said recycling mixture of1,1,1-trifluoro-2-chloroethane, hydrogen fluoride, and optionallytrichloroethylene, is discharged in the form of gas from the middle partof the tower at a temperature which is above the boiling point of theazeotropic mixture of 1,1,1-trifluoro-2-choroethane and hydrogenfluoride and at least 5° C. below the boiling point of anhydroushydrogen fluoride under the internal pressure in the distillation towerand returned to the reaction step (a).
 2. A process as claimed in claim1, wherein the pressure in the distillaation tower is from atomosphericpressure to 20 atm.
 3. A process as claimed in claim 1, wherein amixture of hydrogen fluoride and water is discharged as liquid from thebottom of the distillation tower.
 4. A process as claimed in claim 1,which further comprises adjusting a ratio of1,1,1-trifluoro-2-chloroethane to hydrogen fluoride to be returned tothe reaction process by adding hydrogen fluoride to the distillationtower.
 5. A process as claimed in claim 1, wherein1,1,1-trifluoro-2-chloroethane is discharged as an azeotropic mixture of1,1,1-trifluoro-2-chloroethane and hydrogen fluoride at an upper part ofthe distillation tower above the part at which the1,1,1-trifluoro-2-chloroethane and hydrogen fluoride recycling gas ofstep (b) is discharged.